During the night shift from 09/07/98 to 10/07/98, an unexpected crystalline residue precipitated during an adiabatic diazotization. The personnel considered this residue to be an impurity of the furnished amine, which would have to be removed by clarification before coupling. Therefore, according to the process, a suspension was conveyed over a clarification press to the coupling kettle. Since the press was full, the content (about 50%) was transferred into a vat. The remaining 50% was filtered at the beginning of the morning shift, and remained in the clarification press. During the morning shift, superiors found that the precipitate was not the expected residue of the amine, containing sulphur, but a large quantity of the diazo. Therefore, immediately instructions were issued to wash the coupling kettle and the clarification press with water, and to soak with water the transferred residue. During the inspection of the clarification press at 7:34 hrs, about 2 kg of the material fell to the ground. Since dry diazonium salts decompose immediately in case of mechanical blows or friction, it must be assumed that a part of the residue decomposed because of mechanical stress, which caused as a consequence the explosion of the main part of the residue in the tub.

The explosion caused one fatality, and six persons were injured; 5 persons suffered a temporary shock. The local alarm was sounded and the building evacuated. The on-site fire fighting team was alerted and other buildings in the neighbourhood were evacuated. The event caused material damage for 3 M euros.

The clarification filtration of the diazonium salt and similar processes will be stopped. The periodical training sessions of the employees shall be further improved thematically and didactically. This event should be considered as a warning against the risks involved in the handling of diazonium salt solutions. The explosion risk arising from crystallisation must be particularly emphasised.